Geology of the Southern Dora-Maira Massif: Insights from a Sector with Mixed Ophiolitic and Continental Rocks (Valmala Tectonic Unit, Western Alps)

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Geology of the southern Dora-Maira Massif: insights from a sector with mixed ophiolitic and continental rocks (Valmala tectonic unit, Western Alps)

Gianni Balestro , Francesco Nosenzo , Paola Cadoppi , Gianfranco Fioraso , Chiara Groppo & Andrea Festa

To cite this article: Gianni Balestro , Francesco Nosenzo , Paola Cadoppi , Gianfranco Fioraso , Chiara Groppo & Andrea Festa (2020) Geology of the southern Dora-Maira Massif: insights from a sector with mixed ophiolitic and continental rocks (Valmala tectonic unit, Western Alps), Journal of Maps, 16:2, 736-744 To link to this article: https://doi.org/10.1080/17445647.2020.1824825

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Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=tjom20 JOURNAL OF MAPS 2020, VOL. 16, NO. 2, 736–744 https://doi.org/10.1080/17445647.2020.1824825

Science Geology of the southern Dora-Maira Massif: insights from a sector with mixed ophiolitic and continental rocks (Valmala tectonic unit, Western Alps) Gianni Balestro a, Francesco Nosenzob , Paola Cadoppia, Gianfranco Fiorasoc, Chiara Groppoa and Andrea Festaa aDepartment of Earth Sciences, University of Torino, Torino, Italy; bDepartment of Geological Sciences, Stockholm University, Stockholm, Sweden; cInstitute of Geoscience and Earth Resources, National Research Council, Torino, Italy

ABSTRACT ARTICLE HISTORY In the Valmala sector of the southern Dora Maira Massif (Western Alps), two diﬀerent eclogite- Received 16 April 2020 and blueschist-facies units (i.e. the Rocca Solei and Dronero units, respectively), are separated Revised 8 September 2020 by a shear zone (i.e. the Valmala Tectonic Unit), which peculiarly consists of mixed slices of Accepted 14 September ophiolitic and continental rocks. A detailed geological map at 1:10,000 scale allowed to 2020 ‘ ’ point out that the tectonic slices within the Valmala Tectonic Unit consist of native rock KEYWORDS ‘ ’ slices wrenched from the overlying Dronero Unit, and exotic rocks likely sourced from Alpine tectonics; Dora-Maira other units of the Dora Maira and from a continental margin and an oceanic basin. On the Massif; tectonic mélanges; contrary, rock slices sourced from the underlying Rocca Solei Unit are lacking. The overall ophiolites; shear zone tectonic stack results after an early subduction-related deformation phase (i.e. the D1), and the pervasive overprinting of two subsequent exhumation-related deformation phases (i.e. the D2 and D3). The Valmala Tectonic Unit is inferred to have played a role in decoupling the southern Dora Maira Massif during subduction, and/or in driving exhumation of the ultra-high pressure rocks occurring in the adjoining Brossasco-Isasca Unit.

1. Introduction stratigraphic and structural setting of the whole DM are scattered and actually not exhaustive. The Western Alps represent one of the more studied Throughout a detailed geological map at 1:10,000 scale orogens in the world. Nevertheless, a large part of (see Main Map), this paper provides new detailed infor- this orogen lacks of detailed geological maps and the mation about the geology of a poorly known tectonic official Geological Maps of Italy at 1:100,000 scale stack in the southern DM (i.e. the Valmala sector; Figure are older than 50 years. 1), which lies above the coesite-eclogite bearing Brossaco- During the last tens of years, several works based on Isasca Unit (Chopin et al., 1991; Kienastetal.,1991). In a detailed geological mapping, clearly documented that this sector, two different DM units (i.e. the eclogite-facies the Western Alps are characterized by complex struc- Rocca Solei Unit and the blueschist-facies Dronero Unit) tural and tectono-stratigraphic settings, part of which are separated by a tectonic unit (i.e. the here defined Val- results from pre-orogenic structural inheritances (see mala Tectonic Unit), which peculiarly consists of mixed e.g. Ballèvre et al., 2018; Bell & Butler, 2017; Festa slices of ophiolitic and continental rocks. et al., 2020; Mohn et al., 2011; Tartarotti et al., 2017, and reference therein), superposition of subduction- and exhumation-related deformation stages (see e.g. 2. Regional setting Federico et al., 2015; Manzotti et al., 2014; Roda et al., 2018, and reference therein) and late strike-slip to The Western Alpine orogenic belt developed as a result extensional tectonics (see e.g. Balestro et al., 2009; Per- of the collision between Adria, in the upper plate, and rone et al., 2010, and reference therein). This is the case Europe, in the lower plate, after the closure of the inter- of the Dora-Maira Massif (DM hereafter), a terrane of posed Ligurian–Piedmont oceanic basin (see e.g. Cow- continental crust, worldwide known for the first dis- ard & Dietrich, 1989; Rosenbaum & Lister, 2005; covery of coesite-bearing ultra-high pressure mineral Schmid et al., 2017, and references therein). The belt assemblages (Chopin, 1984). Despite several meta- evolved through the (i) Late Cretaceous to Middle morphic and petrological studies focused on the Eocene subduction, (ii) Late Eocene to Early Oligocene ultra-high pressure Brossasco-Isasca Unit (see e.g. continental collision, and (iii) Late Oligocene to Neo- Compagnoni et al., 2012; Ferrando et al., 2017, and gene deep crust/mantle indentation (see e.g. Solarino reference therein), published data about the tectono- et al., 2018, and references therein). The DM represents

CONTACT Gianni Balestro [email protected] Department of Earth Sciences, University of Torino, Via Valperga Caluso 35, 10125 Torino, Italy © 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of Journal of Maps This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrest- ricted use, distribution, and reproduction in any medium, provided the original work is properly cited. JOURNAL OF MAPS 737

Figure 1. Tectonic map of the Western Alps (modified after Balestro et al., 2015). The red square indicates the study area. a slab of the paleo-European continental margin, which Sampeyre units (Chopin et al., 1991; Groppo et al., was deeply involved in the subduction- and exhuma- 2019), consisting of polymetamorphic mica schists, tion-related Alpine tectonometamorphic processes. It monometamorphic meta-intrusive bodies, Permian is now stacked in the inner sector of the Western metasediments locally with interlayered metavolcanic Alps (i.e. the Upper Penninic Domain; Dal Piaz et al., rocks and Lower Triassic siliciclastic metasediments 2003), and tectonically overlain by different meta- (Boniolietal.,1992;Michard&Vialon,1966).Inbetween ophiolite complexes (see e.g. Balestro et al., 2014, 2018). the eclogite-facies units and the blueschist-facies Dro- The DM consists of different tectonic units (i.e. the nero and Sampeyre units, meta-ophiolitic rocks consist- different ‘Ensembles’ of Vialon, 1966, and lithostrati- ing of serpentinite, metabasite and calcschist graphic units of Sandrone et al., 1993), which differ discontinuously occur (i.e. the ‘ophiolitiferous band’ of from each other for both their lithostratigraphy, Alpine Henry et al., 1993;seealsoCarraro et al., 1971). In the metamorphic P-T peaks and structural positions (Figure southernmost sector of the DM, the Dronero and Sam- 1). The lowermost DM tectonic unit corresponds to the peyre units are tectonically overlain by a succession of PineroloUnit,atectonicwindowoccurringinthecentral carbonate metasediments (pre-Piedmont Auct.; sector of the DM. It was metamorphosed under blues- Michard, 1967), which was detached from the European chist-facies peak conditions (Avigad et al., 2003; Borghi distal margin (Figure 1). This succession mainly consists etal.,1984),anditconsistsofUpperCarboniferousmeta- of Middle-Late Triassic platform-derived dolomitic sediments,whichwereintrudedbyLowerPermiandiori- marble,andofEarly-Jurassicsyn-riftmarbleandcarbon- tic and granitic melts (Bussy & Cadoppi, 1996; Manzotti ate-rich calcschist with layers of carbonate metabreccia. etal., 2016; Perrone et al., 2011). The Pinerolo Unit istec- The structural setting and tectonic evolution of the tonically overlain by different eclogite-facies tectonic southern DM is related to three main stages (see e.g. units (see e.g. Compagnoni & Rolfo, 2003; Gasco et al., Henry et al., 1993), which correspond to the D1, D2 2011), consisting of a composite polymetamorphic base- and D3 phases. The subduction-related D1 was coeval ment, intruded by Lower Permian granitoid bodies, and to the high-pressure and ultra-high pressure meta- of discontinuous monometamorphic successions of sili- morphism, whereas the exhumation-related D2 and ciclasticandcarbonatemetasediments,PermiantoTrias- D3 were coeval to the widespread greenschist-facies sic in age (Bussy & Cadoppi, 1996; Sandrone et al., 1993). retrograde recrystallization (see e.g. Groppo et al., In the southern sector of the DM, these units correspond 2007; Henry et al., 1993). from bottom to top to the eclogite-facies San Chiaffredo Unit, the coesite-eclogite facies Brossasco-Isasca Unit 3. Methods and the eclogite-facies Rocca Solei Unit (Compagnoni et al., 2012). These eclogite-facies units are in turn tecto- The here presented map (Main map) is the result of nically overlain by the blueschist-facies Dronero and fieldwork carried out at 1:10,000 scale and 738 G. BALESTRO ET AL. encompasses an area of around 10 km2 south of the are polymetamorphic rocks (i.e. rocks metamor- Varaita River (i.e. the Valmala Valley). Data were phosed during the Variscan and Alpine orogenic stored in a GIS database (coordinate system WGS 84 cycles): (i) the occurrence of at least two different UTM Zone 32N) and represented on a vector topo- generations of garnet, either porphyroblastic or neo- graphic map compiled from the Carta Tecnica Regio- blastic, with neoblastic garnet growing at the rim of nale Vettoriale of the Regione Piemonte (vector_10 the porphyroblasts and in the matrix; (ii) the occur- series, Edition 1991-2005). Lithostratigraphic analyses rence of sagenitic rutile included in the neoblastic have been supported by microscope thin section garnets, which suggests a garnet growth at the observations. The structural setting is also documen- expense of high-temperature Ti-rich biotite (e.g. ted in a NNW-SSE striking cross section. Regional Spiess et al., 2007). The rare occurrence of Variscan deformation phases have been distinguished through staurolite in the mineral assemblage of these mica overprinting relationships among structures, which schists, was described by Chopin et al. (1991), are represented through stereographic projections further supporting the hypothesis of a polymeta- (equal-area lower-hemisphere). The geological map morphic nature for this lithological unit. is part of a wider project, which aims to produce Meters-thick massive bodies of garnet-bearing detailed geological maps in tectonically meaningful blue amphibole- metabasite (Db in the Main sectors of the Western Alps (Balestro et al., 2011, Map), discontinuously occur along the contact 2013; Barbero et al., 2017; Cadoppi, Camanni, Balestro between the polymetamorphic mica schist and the & Perrone, 2016; Fioraso, Balestro, Festa & Lanteri, gneiss (see below). The fine-grained texture of the 2019; Ghignone et al., 2020). metabasite, its stratigraphic position and relationships with the mica schist (Figure 2(b)), and the lack of any Variscan-related mineral or textural 4. Lithostratigraphy relics, suggest an origin from aphyric mafic to inter- The map includes two DM units, which correspond to mediate volcanic rocks of post-Variscan (Early Per- the Rocca Solei Unit and the Dronero Unit, and the mian?) age. interposed Valmala Tectonic Unit. These units are The gneiss (Dg in the Main Map), up to hun- locally covered by Quaternary alluvial, landslide and dreds of meters in thickness, consists of (i) micro- debris flow deposits (see Main Map). augen gneiss with millimeter-sized K-feldspar porphyroclasts, (ii) fine-grained K-feldspar bearing leu- 4.1. Rocca Solei Unit cogneiss (Figures 2(c) and 3(b)), (iii) layers of quartz- and phengite-bearing mylonitic schist and The RSU occurs in the northern sector of the study (iv) masses of coarse-grained augen gneiss with area (see Main Map) and consists of massive coarse- centimeter-sized K-feldspar porphyroclasts. The het- grained augen gneiss (Rg in the Main Map), with erogeneity of this gneissic succession and the par- centimeters-sized K-feldspar porphyroclasts. The tially preserved primary textures suggest an origin augen gneiss locally preserves microstructural relics from both volcanic, subvolcanic and plutonic acidic of a granitic protolith and it is similar to the Early Per- rocks, which can be related to the Early Permian mian metagranite (275 Ma; Gebauer et al., 1997), magmatic activity which occurred throughout the occurring further north the study area in the Bros- paleo-European crust of the Western Alps (see e.g. sasco-Isasca Unit (Biino & Compagnoni, 1991). Dallagiovanna et al., 2009). The occurrence of Toward the top of the Unit (i.e. at the tectonic contact coarse-grained augen gneiss particularly suggests with the Valmala Tectonic Unit), the augen gneiss is that part of the gneiss succession derive from gran- gradually transformed into mylonitic gneiss (Figure ite bodies. Metagranitoid rocks and orthogneiss of 2(a)), which is several meters to few tens of meters subvolcanic origin emplaced at shallow levels, have thick. been described further south of the study area by Balestro et al. (1995) and by Vialon (1966), 4.2. Dronero Unit respectively.

The DU consists of mica schist, metabasite and 4.3. Valmala Tectonic Unit gneiss. The garnet- and chloritoid-bearing mica schist (Dm in the Main Map; see also Figure 2(b)), The VTU represents a shear zone, consisting of tecto- shows a characteristic texture deﬁned by alternating nically juxtaposed slices of diﬀerent lithologies, both quartz-rich and phengite-rich centimeters-thick ‘exotic’ and ‘native’ in origin (see below), ranging in domains (Figure 3(a)), and it is locally interbedded size from few tens of meters to several hundreds of (NE of Santuario di Valmala) by garnet-bearing meters and lenticular in shape. According to the ter- quartzite layers, up to few meters in thickness. minology used for mélange rock units (see Festa Two lines of evidence suggest that these mica schists et al., 2019 and reference therein), we use the term JOURNAL OF MAPS 739

Figure 2. Field images of different lithologies exposed in Valmala: (a) mylonitic augen gneiss (Rocca Solei Unit, N of Vacot, 44° 33′33, 7°21′22); (b) stratigraphic contact relationships (dashed white line) between the garnet- and chloritoid-bearing mica schist (Dm) and the fine-grained massive metabasite (Db) (Dronero Unit, N of Santuario di Valmala, 44°31′7, 7°20′31); (c) layer of leucogneiss (lg) interbedded in micro-augen gneiss (ag) (Dronero Unit, S of Chiot del Bosco, 44°31′57, 7°20′46); (d) grayish mica schist with millimeters-thick quartz veins (Valmala Tectonic Unit, Bric Guardia, 44°32′50, 7°19′35); (e) massive meta-quartzdiorite with centimeters-thick mafic enclave (me) and acidic dyke (ad) (Valmala Tectonic Unit, Comba di Valmala, 44°32′54, 7°20′27); (f) carbonate-rich calcschist (Valmala Tectonic Unit, W of Perotto, 44°32′34, 7°20′17); (g) banded metabasite showing the S1 foliation (dashed white lines) deformed by D2 folds (Valmala Tectonic Unit, E of Rora, 44°32′56, 7°20′54); (h) carbonate-bearing serpentinite (Valmala Tectonic Unit, W of Botto, 44°32′53, 7°19′52).

‘native’ and ‘exotic’ to indicate the lithology of rock . ﬁne-grained mylonitic gneiss and micro-augen slices whose source is present or not in the units gneiss (Vg in the Main Map) with peculiar dark surrounding the VTU (i.e. the RSU and DU), grayish pervasively fractured K-feldspar porphyro- respectively. clasts. These blocks closely resemble the gneiss of ‘Native’ rock slices, which show a close lithological the DU, potentially Early Permian in age; aﬃnity with the DU, consist of the following . grayish mica schist (Vt in the Main Map) with lithologies: widespread quartz veins (Figure 2(d)), chloritoid- bearing mica schist and garnet-bearing mica schist, . garnet- and chloritoid-bearing mylonitic mica with local layers of garnet- and white mica-bearing schist (Vm in the Main Map), which are texturally metabasite, decimeters- to few meters in thickness. and compositionally similar to the polymeta- This mica schist is locally associated with ankerite- morphic mica schist of the DU, although perva- bearing mica schist and reddish mylonitic schist sively deformed and recrystallized; (Vn in the Main Map), and lithologically resembles 740 G. BALESTRO ET AL.

Figure 3. Images of different field-scale structures: (a) D2 crenulation cleavage (dashed black lines) in garnet- and chloritoid-bearing mica schist (Dronero Unit, N of Santuario di Valmala, 44°31′7, 7°20′35); (b) D2 folds deforming the contact between metabasite (Db) and leucogneiss (Dg) (Dronero Unit, E of Santuario di Valmala, 44°30′48, 7°21′2); (c) D3 extensional shear planes (dashed red lines) dragging the S2 foliation (dashed white lines) in mylonitic dioritic gneiss (Valmala Tectonic Unit, Comba di Valmala, 44° 32′48, 7°20′30). (d–h) Equal-area lower-hemisphere stereographic projections of different structures (n: number of data; counting method: Fisher distribution): (d) poles of S1 foliation; (e) poles of D2 axial plane (AP2); (f) D2 axes (A2); (g) poles of S2 foliation; (h) D2 stretching lineations (L2).

the Permian monometamorphic metasediments This lithology shows widespread maﬁc enclaves and described in the southernmost sector of the DU acidic dykes, several centimeters in thickness, and it by Michard and Vialon (1966). can be tentatively compared to the metadiorite exposed in central-northern sector of the DM (San- ‘Exotic’ rock slices, which do not show any litho- drone et al., 1988); logical aﬃnity with units directly surrounding the . quartz-rich schist (Vq in the Main Map) VTU, consist of the following lithologies: mainly shows a mylonitic structure but locally preserves layers of white to pale green quartzite. . garnet- and blue amphibole-bearing meta-quartz- The latter is quite similar to the Early Triassic diorite (Vg in the Main Map), which is mostly quartzite (Werfenian Auct.) widespread in the massive (Figure 2(e)), but transformed into carbon- succession of the DM Sampeyre Unit (Vialon, ate-bearing mylonitic gneiss at its top (Figure 3(c)). 1966); JOURNAL OF MAPS 741

. marble cropping out W of Chiarreri locality (Vd in between the polymetamorphic mica schist and meta- the Main Map), and mostly transformed into tec- volcanic rocks (Dm and Dg in the Main Map) is tonic carbonate breccia; clearly deformed and overturned in the sector between . carbonate-rich calcschist (Vc in the Main Map; see two major fold hinges (i.e. between Chiot Martin and Figure 2(f)) and graphite-rich mylonitic schist; Santuario di Valmala). D2 folds also occur within the . pervasively sheared carbonate-bearing serpentinite VTU (Figure 2(h)), although, in this unit, the D2 is and talc-bearing serpentine schist (Vs in the Main mainly characterized by the occurrence of related tec- Map, see Figure 2(g)), cropping out in the Botto, tonic contacts. The latter bounds both the VTU and Perotto, Rora and Vacot localities; the embedded ‘native’ and ‘exotic’ slices. . banded and massive metabasite (Vb in the Main The D3 is defined only at the mesoscale by open Map, see Figure 2(h)), which includes garnet- and folds, deforming the S2 within the mica schist of the blue amphibole in its mineral assemblage, and it VTU and DU (Vt, Vm and Dm in the Main Map). is locally transformed into carbonate-bearing D3 axial planes are E to ENE dipping at a high angle greenschist and coarse-grained recrystallized meta- and the D3 axes mainly plunge toward NE at low to basite. This metabasite does not retain textural or medium angle. The D3 is also characterized by the mineralogical relics of its protolith. However, its development of discrete shear planes, about SSE-dip- spatial relationship with the serpentinite suggests ping at a high angle, showing SC fabrics consistent an origin from gabbro. with extensional top-to-S and top-to-SE sense of shears (Figure 3(c)). The D3 shear planes mainly Part of the ‘native’ rock slices (i.e. the garnet- and developed within the VTU, corresponding to major chloritoid-bearing mica schist and the gneiss) are faults at the map scale. The D3 deformation regionally mostly distributed in the uppermost part of the accommodated the development of exhumation- VTU, along the tectonic boundary with the DU. Calcs- related dome-like structure of the DM (Lardeaux chist, serpentinite and metabasite ‘exotic’ slices are et al., 2006), during which pre-D3 structures were mainly aligned in the central sector of the VTU, reoriented. whereas slices of meta-quartzdiorite are clearly localized in its lowermost part, along the tectonic bound- 6. Conclusion ary with the RSU. The southern DM consists of eclogite and coesite- eclogite facies units, which are tectonically overlain 5. Structures by blueschist-facies units.Ournewgeologicalmap Three main deformation phases (i.e. D1, D2 and D3) (seeMainmap)showsthatthistectonometamorphic have been distinguished (see Main Map). setting matches well with a complex inherited tec- The D1 phase is defined by an early foliation (i.e. tono-stratigraphic setting. The lithostratigraphy of the S1), which is preserved in more massive rocks the RSU and DU, which are tectonically separated such as the metabasite (Figure 2(h)) and meta-quartz- through the VTU, is not comparable (see also Com- diorite of the VTU (Vt and Vr in the Main Map), and pagnoni et al., 2012). In the VTU, which consists of the metabasite and coarse-grained augen gneiss of the the tectonic mixing of different rock slices, ‘native’ DU (Db and Dg in the Main Map). The S1 is scattered rock slices of polymetamorphic mica schist, orthog- along a NNE-striking best fit great circle (Figure 3(d)), neiss and, possibly, monometamorphic mica schist, which is roughly coherent with reorientations related seem to be wrenched from the overlying DU to the D2 phase. The latter is characterized by the sequence. On the contrary, lithological analogs of development of tight to close folds (Figure 3(b,c)), the quartz-rich schist and meta-quartz diorite rock which deformed S1, and a pervasive axial plane foli- slices do not occur in the units surrounding the ation (i.e. the S2) in less massive rocks such as mica VTU (see Sandrone et al., 1993). Carbonate- and schist and calcschist of the VTU (Vt and Vn in the mafic-ultramafic rock slices, could be wrenched Main Map) as well as mica schist of the DU (Dm in from tectonic units now overlying the DM (see Bales- the Main Map). D2 axial planes dips toward ESE at tro et al., 2019), although different sources cannot be low to medium angle (Figure 3(e)), whereas D2 axes excluded. Further investigations are necessary to bet- are on average E-plunging at a low angle (Figure 3 ter constraints their origin and age. (f)). Because of refraction along fold limbs, the distri- The RSU, VTU and DU seem to share the same bution of S2 (Figure 3(f)) is more scattered than that of exhumation-related D2 and D3 deformation. On the D2 axial planes. Stretching lineations on S2 are almost contrary, subduction-related D1 deformation devel- ENE-plunging at low angle (Figure 3(h)), highlighting oped differently in the RSU and DU, which show that D2 folds are geometrically non-cylindrical. At the different metamorphic P-T peaks (Groppo et al., map scale, the effect of D2 folds is more evident in the 2019). Although detailed petrologic data on the VTU DU, wherein the originally S1-parallel contact are currently lacking and most lithologies are strongly 742 G. BALESTRO ET AL. retrogressed under greenschist-facies conditions, the Funding high pressure mineral assemblages preserved as relics This work was supported by research grants from Università in the less retrograded lithologies seem to support di Torino, Ricerca Locale ‘ex 60%’ 2017-2018-2019 (AF, blueschist-facies peak metamorphic conditions, while GB, PC) and from the Italian Ministry of University and eclogite-facies mineral assemblages have not been Research, ‘Finanziamento annuale individuale delle attività ’ observed so far. This suggests that the VTU may base di ricerca 2017 (GB, AF). have shared part of the subduction path and related tectonic history with the DU. Tectonic wrenching of ORCID part of its lithostratigraphic sequence, probably occurred at this stage, forming the early VTU Gianni Balestro http://orcid.org/0000-0001-5215-4659 internal architecture. Subduction-related processes seem to represent the most suitable mechanism References responsible for the wrenching of tectonic slices, as documented in other sectors of the Western Alps Avigad, D., Chopin, C., & Le Bayon, R. (2003). Thrusting and Northern Apennines (see e.g. Barbero et al., and extension in the southern Dora-Maira ultra-high pressure massif (Western Alps): View from below the 2020; Roda et al., 2020). However, the inner setting coesite-bearing unit. Journal of Geology, 111(1), 57–70. of the VTU was reorganized during D2 and D3 https://doi.org/10.1086/344664 phases. The effect of this exhumation-related tec- Balestro, G., Cadoppi, P., Di Martino, L., & Sacchi, R. tonics was particularly pervasive within the VTU, (1995). Il settore meridionale del Massiccio Dora-Maira which likely marked the tectonic contact between (Valli Maira e Varaita): Inquadramento, carta geologica e guida a un’escursione. Accademia Nazionale Delle eclogite- and blueschist-facies units. This type of tec- – fi Scienze, 14, 501 529. tonic contact corresponds to a rst-order shear zone Balestro, G., Cadoppi, P., Perrone, G., & Tallone, S. (2009). along the Western Alps, where it has been inter- Tectonic evolution along the Col del Lis-Trana defor- preted as the boundary between two orogen-scale mation zone (internal Western Alps). Italian Journal of domains (i.e. the ‘eclogite belt’ and ‘frontal wedge’ Geosciences, 128(2), 331–339. https://doi.org/10.3301/ of Malusà et al., 2011). IJG.2009.128.2.331 Balestro, G., Festa, A., Borghi, A., Castelli, D., Tartarotti, P., Finally, it is worth noting that just to E of the & Gattiglio, M. (2018). Role of Late Jurassic intra-oceanic studied sector, the RSU is not continuous, and rocks structural inheritance in the Alpine tectonic evolution of pertaining to the VTU seem to be directly juxtaposed the Monviso meta-ophiolite complex (Western Alps). to the ultra-high pressure Brossasco-Isasca Unit (see Geological Magazine, 155(2), 233–249. https://doi.org/ Compagnoni et al., 2012). The VTU may thus have 10.1017/S0016756817000553 had also a role in decoupling the southern DM during Balestro, G., Festa, A., & Dilek, Y. (2019). Structural architecture of the Western Alpine Ophiolites, and the subduction and/or in driving exhumation of ultra- Jurassic seafloor spreading tectonics of the Alpine high pressure rocks. Tethys. Journal of the Geological Society, 176(5), 913– 930. https://doi.org/10.1144/jgs2018-099 Balestro, G., Festa, A., Dilek, Y., & Tartarotti, P. (2015). Software Pre-Alpine extensional tectonics of a peridotite localized oceanic core complex in the late Jurassic, The topographic map and the geological map database high-pressure Monviso ophiolite (Western Alps). were edited with QGIS (v. 3.12.1-București), while the Episodes, 38(4), 266–282. https://doi.org/10.18814/epiiugs/ final map layout was drawn with Adobe® Illustrator® 2015/v38i4/82421 CS5. Structural data have been projected using Open Balestro, G., Fioraso, G., & Lombardo, B. (2011). Geological Stereo. map of the upper Pellice Valley (Italian Western Alps). Journal of Maps, 7(1), 634–654. https://doi.org/10.4113/ jom.2011.1213 Balestro, G., Fioraso, G., & Lombardo, B. (2013). Geological Acknowledgements map of the Monviso massif (Western Alps). Journal of – We thank A. Merschat for editorial handling and com- Maps, 9(4), 623 634. https://doi.org/10.1080/17445647. ments, and T. Hirajima, H.P. Schertl and T. Pingel for 2013.842507 their critical and thorough reviews. This work was sup- Balestro, G., Lombardo, B., Vaggelli, G., Borghi, A., Festa, ported by research grants from Università di Torino, A., & Gattiglio, M. (2014). Tectonostratigraphy of the Ricerca Locale ‘ex 60%’ 2017–2018–2019 (AF, GB, PC), northern Monviso meta-ophiolite complex (Western – and from the Italian Ministry of University and Research, Alps). Italian Journal of Geosciences, 133(3), 409 426. ‘Finanziamento annuale individuale delle attività base di https://doi.org/10.3301/IJG.2014.13 ricerca’ 2017 (GB, AF). Ballèvre, M., Manzotti, P., & Dal Piaz, G. V. (2018). Pre- Alpine (Variscan) inheritance: A key for the location of the future Valaisan basin (Western Alps). Tectonics, 37 – Disclosure statement (3), 786 817. https://doi.org/10.1002/2017TC004633 Barbero, E., Festa, A., Fioraso, G., & Catanzariti, R. (2017). No potential conflict of interest was reported by the author Geology of the Curone and Staffora Valleys (NW Italy): (s). 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